An extrusion die is used to extrude molten thermoplastic material into a relatively thin film or sheet. Conventional extrusion dies have a die cavity with the general shape of a coat hanger. These extrusion dies have been generally referred to a coat hanger dies. Typical coat hanger dies include an inlet, an inlet manifold, a generally triangular shaped preland portion, a melt well, and a die exit or gap. In these dies, a back edge of the preland portion includes linear edges that form a taper converging towards a die entrance. The preland portion provides a resistance to flow that varies over the width of the die to uniformly spread the thermoplastic material across the entire die.
The rheological characteristics of the molten thermoplastic materials and the pressure these thermoplastic materials exert on the die bodies result in clamshelling, or non-uniform deflection. Clamshelling results in uneven flow of the thermoplastic material through the die exit. These or other nonuniform patterns are undesirable for the production of flat sheet materials. The nonuniformities can be corrected by adjusting the die lip exit gap. Most extrusion dies are equipped with some form of lip adjustment mechanism to adjust the gap. If a particular die is to used to form a large quantity of a single layer of material, and the extrusion process is not subject to any interruptions, the adjustment of the gap is an acceptable method of removing or at least addressing these nonuniformities. However, since the performance of an extrusion die is influenced by a number of factors including flow rate, temperature, the nature of the thermoplastic materials, and the like, the use of the lip adjustment mechanism to remove or minimize these nonuniformities is not practical where a die is to be used for short runs to produce different products. For each production change the die lips must be the empirically adjusted to the new conditions. This results in a loss of production time and a waste of material as off specification product is produced during start up.
The nonuniformity that is created in these prior die designs is more serious when two or more layers of a thermoplastic material are to be coextruded. In these cases, the formation of the M or W shaped boundaries between the co-extrusion layers creates an interfacial deformation that cannot be corrected by adjusting the die exit gap using the lip adjustment mechanism.
In the past, there have been proposals to address these problems mostly through the design of different manifold configurations. In one such design, the back line of the preland portion is structured to be a uniform distance from the die exit. While this particular design does minimize the above problems, there are concerns relative to maintaining the flow through the die without creating areas of substantially higher residence time that over time could lead to the degradation of the thermoplastic material.